(1) Field of the Invention
The present invention relates to a radar apparatus used at high frequencies such as a microwave band and a millimeter waveband, and in particular, to a radar apparatus which requires higher performance and lower power consumption.
(2) Description of the Related Art
In recent years, an in-vehicle radar apparatus is required to detect an object (hereinafter, referred to as an obstacle) with high accuracy. In particular, in the in-vehicle radar apparatus, higher performance is required of a transmitter for emitting a radio wave to the obstacle and a receiver for receiving and detecting the radio wave reflected from the obstacle, since they use a high-frequency signal.
As for the in-vehicle radar apparatus in particular, a reflected wave from the obstacle has a wide dynamic range from a weak level to a strong level. Therefore, It is important that high-frequency signal isolation is high between the transmitter and the receiver. When the high-frequency signal leaks between the transmitter and the receiver, a noise signal is included in a signal to be transmitted or a signal to be received so that accurate transmission and reception become infeasible. Furthermore, there is a possibility that an unnecessary signal may block obstruct communication and cause interference with the radio wave, which may result in a malfunction.
FIG. 1 is a diagram showing a configuration of a first radar apparatus in a conventional form. As shown in FIG. 1, a radar apparatus 10 is a radar apparatus of a spread spectrum type using a pseudo-noise code (hereinafter, referred to as a PN code).
The high-frequency signal outputted from an oscillator 12 is divided to a balanced modulator 13 and a quadrature demodulator 22a. Here, the high-frequency signal divided to the balanced modulator 13 is a transmission local oscillation signal. The high-frequency signal divided to the quadrature demodulator 22a is a reception local oscillation signal.
The PN codes outputted from a PN code generator 15 are divided to a balanced modulator 14 and a balanced modulator 20 via a variable delay device 21. Here, the PN code divided to the balanced modulator 14 is a transmission PN code. The PN code divided to the balanced modulator 20 via the variable delay device 21 is a reception PN code.
The transmission local oscillation signal and an intermediate-frequency signal outputted from an intermediate-frequency oscillator 11 are mixed by the balanced modulator 13 to be outputted as a modulation signal. The transmission PN code and the modulation signal outputted from the balanced modulator 13 are mixed by the balanced modulator 14 to be outputted as a transmission signal.
The transmission signal outputted from the balanced modulator 14 is transmitted as a radar wave from a transmitting antenna 17 via a transmission signal band pass filter 16. The radar wave transmitted from the transmitting antenna 17 is reflected by the obstacle. The reflected wave which is obtained by being reflected by the obstacle is received as a reception signal by a receiving antenna 18.
The reception signal received by the receiving antenna 18 is amplified by a low noise amplifier 19 and outputted as an amplified signal. The reception PN code and the amplified signal outputted from the low noise amplifier 19 are mixed by the balanced modulator 20 and outputted as a correlated signal. The correlated signal outputted from the balanced modulator 20 is divided to the quadrature demodulator 22a and a quadrature demodulator 22b. Here, the correlated signal divided to the quadrature demodulator 22a is a first correlated signal. The correlated signal divided to the quadrature demodulator 22b is a second correlated signal.
The reception local oscillation signal is divided to the quadrature demodulator 22a and the quadrature demodulator 22b via a 90-degree phase shifter 23. Here, the reception local oscillation signal divided to the quadrature demodulator 22a is a first reception local oscillation signal. The reception local oscillation signal divided to the quadrature demodulator 22b via the 90-degree phase shifter 23 is a second reception local oscillation signal.
The first reception local oscillation signal and the first correlated signal are mixed by the quadrature demodulator 22a and outputted as an I (in-phase) signal. The I (in-phase) signal outputted from the quadrature demodulator 22a is outputted via an intermediate-frequency band pass filter 24a and a logarithmic amplifier 25a. 
The second reception local oscillation signal and the second correlated signal are mixed by the quadrature demodulator 22b and outputted as a Q (quadrature) signal. The Q (quadrature) signal outputted from the quadrature demodulator 22b is outputted via an intermediate-frequency band pass filter 24b and a logarithmic amplifier 25b. 
The radar apparatus 10 can obtain the reflection intensity by performing signal processing on the I (in-phase) signal outputted from the logarithmic amplifier 25a and the Q (quadrature) signal outputted from the logarithmic amplifier 25b. 
Here, double-balanced mixers such as Gilbert cells are used for the balanced modulators 13, 14, 20 and the quadrature demodulators 22a, 22b. Thus, a 26 GHz frequency signal source equivalent to a frequency band of a carrier signal has been conventionally used for the oscillator 12 which is a local signal source of the balanced modulator 13 and the quadrature demodulators 22a, 22b. 
FIG. 2 is a diagram showing a configuration of a transmission and reception part of a first radio communication apparatus in a conventional form. As shown in FIG. 2, an oscillator 31 is connected to a transmission frequency conversion unit 33 via a frequency-multiplier-circuit 32. It is further connected to a reception frequency conversion unit 34 via the frequency-multiplier-circuit 32. However, an isolating circuit or an attenuating circuit is not placed in a path which connects the transmission frequency conversion unit 33 to the reception frequency conversion unit 34. The isolating circuit or the attenuating circuit prevents a signal caused by the local oscillation signal from flowing from the transmission frequency conversion unit 33 to the reception frequency conversion unit 34. Similarly, an isolating circuit or an attenuating circuit for preventing a signal caused by the local oscillation signal from flowing from the reception frequency conversion unit 34 to the transmission frequency conversion unit 33 is also not placed. For this reason, a signal caused by the local oscillation signal leaks from a transmitting side to a receiving side (see for example Japanese Unexamined Patent Application Publication No. 2003-229722).
In comparison, FIG. 3 is a diagram showing the configuration of a second radar apparatus in a conventional form. As shown in FIG. 3, an interrupting circuit or an attenuating circuit is placed in a path connecting an up converter 42 to a down converter 43 so that a signal caused by the local oscillation signal outputted from an oscillator 41 is not leaked from the transmitting side to the receiving side. To be more specific, an isolator 44 as the interrupting circuit and an attenuator 45 as the attenuating circuit are placed (see for example Japanese Unexamined Patent Application Publication No. 2000-9829).
FIG. 4 is a diagram showing a configuration of a transmission and reception part of a second radio communication device in the conventional form. As shown in FIG. 4, in a transmitter-receiver used for the radio communication device and the like, a signal outputted from an oscillator 51 is inputted to a mixer 53 via a multiplier 52a of a multiplication rate of A times. It is inputted to a modulator 54 via a multiplier 52b of a multiplication rate of B times (see for example Japanese Unexamined Patent Application Publication No. 2001-44880). However, higher-frequency characteristics such as gain characteristics of the multipliers cannot be guaranteed just by diverting the multipliers 52a and 52b used in the radio communication device to the radar apparatus which uses a higher-frequency signal than the signal used by the radio communication device. Therefore, a scheme which conforms with the radar apparatus is required.
In the case of the first conventional radar apparatus, however, a high-frequency signal equivalent to the frequency band of a carrier wave (such as 26 GHz) is outputted as-is from the oscillator 12 which is the signal source of the local oscillation signal of the balanced modulator 13 and the quadrature demodulators 22a, 22b. 
In this case, it is not possible to secure sufficient isolation from the high-frequency signal outputted from the oscillator 12 in the path connecting the balanced modulator 13 to the quadrature demodulator 22a. For this reason, a signal caused by the high-frequency signal outputted from the oscillator 12 leaks from the balanced modulator 13 to the quadrature demodulator 22a. As a result, there is a problem that wrong detection occurs due to the leaked signal, and the accurate distance and location of the obstacle cannot be identified from the reflected wave obtained through the reflection of the radar wave from the obstacle.
As for the second conventional radar apparatus, the isolator 44 and attenuating circuit 45 are placed in the path connecting the up converter 42 to the down converter 43 so that a signal caused by the local oscillation signal outputted from the oscillator 41 is not leaked from the transmitting side to the receiving side. In this configuration, it is necessary, for the sake of realizing an input level required for the availability of the up converter 42, to consider losses caused by the isolator 44 and attenuating circuit 45 as against the local oscillation signal outputted from the oscillator 41. Similarly, it is necessary, for the sake of realizing an input level required for the availability of the down converter 43, to consider the losses caused by the isolator 44 and attenuating circuit 45 as against the local oscillation signal outputted from the oscillator 41. For this reason, it is necessary to output the local oscillation signal from the oscillator 41 with its output level rather high. Thus, there are problems that operating currents increase, power supply voltage needs to be increased, and power consumption becomes high.